1. Field of the Invention
The invention relates to the production of pollutant sorbents, the reduction of pollutants, and energy providing fuels. Generally, the invention relates to the use of oil shale and oil shale products as sorbents or reductants for reducing pollutants, and, in particular, to the thermal processing of oil shale to produce pollutant absorbing and adsorbing solids and combustible or reducing gases.
2. State of the Art
Coal, oil, natural gas, oil shale, oil sands, and other carbon-containing fuel feedstock materials (for example: forestry industry products, byproducts, and residues; agriculture crops, byproducts, and residues; animal wastes and carcasses; municipal solid waste, sewage sludge solids, construction and demolition debris, waste tires, and other forms of refuse-derived fuel) can be converted from chemical potential energy to heat and gaseous products that can be used to generate electrical power, or to produce higher value chemicals and components.
In the context of this document, the term “thermal conversion” generally implies any gaseous or solid process that liberates or transforms the chemical potential energy into heat, hot gases, combustible gases, combustible liquids, combustible solids such as char, and/or non-combustible solids such as ash or calcined minerals, or any subset of these.
Thermal conversion of such gaseous and solid materials produces various pollutants, such as nitrogen compounds or sulfur compounds, which are believed to be involved in the formation of smog and acid rain. If the fuel includes mercury, the combustion also produces mercury compounds, which have been identified by the Environmental Protection Agency (“EPA”) as a significant toxic pollutant. Air pollutant control legislation, such as the Clean Air Act (“CAA”), the Clean Air Interstate Regulation (“CAIR”), and the Clean Air Mercury Regulation (“CAMR”), regulate emissions of many of the pollutants released from thermal conversion processes.
The composition of the pollutant species produced by thermal conversion processes is a strong function of the availability of oxygen in the process. Under the reducing conditions of pyrolysis and gasification, sulfur bound in the fuel is typically converted to reduced forms of sulfur, such as hydrogen sulfide, carbonyl sulfide, and carbon disulfide. Nitrogen contained in the fuel is converted to reduced nitrogen compounds, including ammonia, hydrogen cyanide, and molecular nitrogen. Most of the mercury in a fuel is converted to volatile elemental mercury (“Hg°”) and speciated mercury, such as mercury chloride (“HgCl2”). Under reducing conditions, phosphorus (“P”) reacts with metals to form phosphate compounds, and may also be converted to phosphine (“PH3”), phosphonium compounds, and other reduced forms of phosphorus. Chlorine (“Cl”) may react with alkali metals (such as sodium and potassium), alkali-earth metals (such as calcium and magnesium), and other metals (such as mercury, zinc, and iron), but it is also converted to diatomic chlorine gas (“Cl2”) and hydrochloric acid gas (“HCl”). Fluorine, bromine, and iodine behave similar to chlorine.
Under the oxidizing conditions of combustion, sulfur bound in the fuel is converted to gaseous sulfur dioxide or sulfur trioxide. These sulfur compounds quickly equilibrate with moisture (“H2O”) to form sulfuric acid (“H2SO4”). Nitrogen bound in the fuel is converted to nitric oxide and nitrogen dioxide or other oxides of nitrogen. Combustion with air also results in nitrogen oxides as a result of high temperature reactions of atomic oxygen (“O”) and hydroxide radicals with molecular nitrogen. Phosphorus, chlorine, fluorine, bromine, and iodine are readily converted to phosphoric acid (“H3PO4”), hydrochloric acid (“HCl”), and hydrofluoric acid (“HF”), bromous acid (“HBrO”), and iodic acid (“HIO3”), as well as other reactive volatile compounds. These acid gases are corrosive to equipment used in combustion processes, such as in a combustion device or in boiler tubes in a combustor. Therefore, it is desirable to limit the formation of the acid gases or to remove the acid gases close to their point of generation in a combustion device or thermal conversion process.
Various technologies have been developed to decrease emissions from coal-fired powerplants. Limestone has been used as a sorbent for SOx pollutants, as disclosed in U.S. Pat. No. 3,995,006 to Downs et al., U.S. Pat. No. 5,176,088 to Amrhein et al. (“Amrhein”), and U.S. Pat. No. 6,143,263 to Johnson et al. This technology is known as limestone injection multiple burner (“LIMB”) technology or limestone injection dry scrubbing (“LIDS”) technology. The limestone is injected into a region of a furnace having a temperature of 2,000° F. to 2,400° F.
Limestone (mainly CaCO3) and dolomite (“CaCO3—MgCO3”) and their derivatives have also been shown to react with H2S. Either uncalcined limestone or dolomite, half-calcined dolomite, fully calcined limestone or dolomite, lime, or hydrated lime (“CaOH”) will react to form calcium sulfide (“CaS”) or magnesium sulfide (“MgS”).
Organic and amine reducing agents, such as ammonia or urea, are used to selectively reduce NOx pollutants, as disclosed in U.S. Pat. No. 3,900,554 to Lyon. This technique is known as selective noncatalytic reduction (“SNCR”). The reducing agent is injected into a furnace at a temperature from about 975 K to about 1375 K so that a noncatalytic reaction selectively reduces the NOx to molecular nitrogen (“N2”). The ammonia is injected into a region of the furnace having a temperature of 1,600° F. to 2,000° F.
The LIMB and SCNR technologies have been combined to simultaneously remove the NOx pollutants and the SOx pollutants. The limestone is used to absorb the SOx pollutants while the ammonia is used to reduce the NOx pollutants. However, this combination technology is expensive to implement and adds increased complexity to the process.
NOx reburning has also been used to remove the NOx pollutants, as disclosed in U.S. Pat. No. 5,139,755 to Seeker et al. In NOx reburning, the coal is combusted in two stages. In the first stage, a portion of the coal is combusted with a normal amount of air (about 10% excess), producing the NOx pollutants. In the second stage, the remaining portion of the coal is combusted in a fuel-rich environment. Hydrocarbon radicals formed by combustion of the coal react with the NOx pollutants to form N2. Fuel/air staging has also been used to reduce the NOx pollutants. Fuel and air are alternately injected into a combustor to provide a reducing zone where the nitrogen in the fuel is evolved, which promotes the conversion of the nitrogen to N2. The air is injected at a separate location to combust the fuel volatiles and char particles. By staging or alternating the fuel and the air, the local temperature and the mixture of air and fuel are controlled to suppress the formation of the NOx pollutants. Fuel/air staging attempts to prevent NOx formation while NOx reburning promotes NOx reduction and destruction.
To absorb mercury or mercury-containing pollutants, activated carbon is used as a sorbent, as disclosed in U.S. Pat. No. 5,827,352 to Altman et al., and U.S. Pat. No. 6,712,878 to Chang et al. The activated carbon is present as a fixed or fluidized bed or is injected into the flue gas.
Oil shale is a sedimentary rock that includes an inorganic matrix of carbonate, oxide, and silicate compounds impregnated with a polymeric material called kerogen. Kerogen is an organic substance that is insoluble in petroleum solvents. When heated, the kerogen pyrolyzes to produce gas, oil, bitumen, and an organic residue. Pyrolyzing the kerogen is also known as retorting. Oil shale also includes carbonate minerals, such as calcium carbonate, and other hydrocarbon materials, such as paraffins, cycloparaffins, aliphatic and aromatic olefins, single ring aromatics, aromatic furans, aromatic thiophenes, hydroxyl-aromatics, dihydroxy aromatics, aromatic pyrroles, and aromatic pyridines, and other polynuclear aromatic hydrocarbons. Oil shale is typically co-located with coal and oil and is found in various regions of the western United States, such as in Utah, Colorado, and Wyoming, and in the eastern United States, such as in Virginia and Pennsylvania. Large deposits of oil shale are also found in Canada, Australia, Europe, Russia, China, Venezuela, and Morocco. Given the abundance of oil shale throughout the world, its value would be significant if beneficial uses are identified and employed. Oil shale utilization has not been presently appreciated due to the high cost of recovering the kerogen from the shale.
When oil shale containing considerable amounts of calcium carbonate is heated, the calcium carbonate undergoes calcination, which is an endothermic reaction in which the calcium carbonate (“CaCO3”) is converted to lime (“CaO”). For each kilogram of calcium carbonate that is calcined, as much as 1.4 MJ to 1.6 MJ (or about 600 British Thermal Units (“BTU”) to 700 BTU per pound mass) of the available heat energy is consumed. This loss of energy translates to a process efficiency penalty when limestone or dolomite is used as an injected sorbent. In the case of oil shale, the kerogen can be oxidized to offset the heat sink associated with carbonate calcination.
To extract energy from the oil shale, the oil shale can be heated in a retorting zone of a fluidized bed reactor vessel to a temperature sufficient to release, but not combust, volatile hydrocarbons from the oil shale, as disclosed in U.S. Pat. No. 4,373,454 to Pitrolo et al. The temperature used in the retorting zone provides minimal calcination of the calcium carbonate. The volatile hydrocarbons flow to a combustion zone of the fluidized bed combustor, where the volatile hydrocarbons are combined with excess air and are combusted. Calcination of the calcium carbonate occurs in the combustion zone. During retorting, nitrogen compounds in the oil shale are converted to NOx compounds and are reduced to nitrogen and water or oxygen by the volatile hydrocarbons.
Oil shale has been used to absorb SO2 and HCl in a circulating fluidized bed, as disclosed in “Combustion of Municipal Solid Wastes with Oil Shale in a Circulating Fluidized Bed,” Department of Energy Grant No. DE-FG01-94CE15612, Jun. 6, 1996, Energy-Related Inventions Program Recommendation Number 612, Inventor R. L. Clayson, NIST Evaluator H. Robb, Consultant J. E. Sinor and in “Niche Market Assessment for a Small-Scale Western Oil Shale Project,” J. E. Sinor, Report No. DOE/MC/11076-2759.
Many of the pulverized coal combustors in operation do not meet the new standards promulgated by the United States Environmental Protection Agency under CAIR and CAMR. Upwards of about 75 percent of all currently existing pulverized coal combustors may have to be phased out or retrofit to satisfy the new pollutant standards.